1. Field of the Invention
This invention relates to oscillator circuits.
More particularly, the invention relates to an inductive sine wave oscillator circuit which includes an operational amplifier and in which the frequency and amplitude of the output signal produced by the circuit varies with and is controlled by the inductance of a coil in the circuit.
In a further respect, the invention relates to an oscillator circuit of the general type described in which the signal that is input to the amplifier undergoes a phase shift as the signal passes through the amplifier.
In another respect, the invention relates to an oscillator circuit of the general type described in which the frequency and amplitude of the output signal for a particular inductance of the coil remains relatively stable and constant while the operating temperature of the coil varies.
In yet still another aspect, the invention relates to an oscillator circuit of the type described in which the coil can be formed from a material having a relatively high electrical resistivity.
2. Description of the Related Art
Oscillators convert direct current to alternating current or other forms of pulsating direct current. The waveforms can be sine wave, square wave, triangle wave, sawtooth wave, or other combinations of these basic wave types. Sine wave oscillators require positive feedback and resistor-capacitor (RC) or inductor-resistor (LR) components. Positive feedback occurs when a portion of the oscillator output signal is fed back to the input of the oscillator and is in phase with the input signal. Conventional oscillators include the RC phaseshift oscillator, Wein bridge oscillator, Armstrong oscillator, Hartley oscillator, Colpitts oscillator, Clapp oscillator, and crystal oscillator.
A Colpitts oscillator produces a sine wave output and includes an LC tank circuit. Colpitts oscillators are often utilized in eddy current proximity measuring transducers, termed PMT's for short. When a Colpitts oscillator is utilized in a PMT, the probe coil of the PMT functions as the inductive element in the LC tank circuit. Varying the distance of the probe coil from an electrically conductive target material varies the inductance and resistance of the coil and consequently varies the amplitude of the output signal. In particular, when the probe coil is positioned proximate a metal, "eddy" current is induced in the metal. The eddy current absorbs power from the LC tank circuit of the oscillator, reduces the loop gain of the oscillator, and reduces the output voltage of the oscillator.
Conventional Colpitts oscillator PMT's utilize an inductive coil which has a high Q value where Q is equal to wL/R and EQU w=2.times.3.14.times.frequency
L=coil inductance PA1 R=coil resistance
Thus, to obtain a high Q value, the probe coil must have a high inductance L and be fabricated from a material having a low electrical resistance R. The operating frequency w must also be high. A low resistance inductive coil can be obtained by utilizing a copper or silver wire. The principal disadvantage of copper and silver coils is that the electrical resistance of copper and silver markedly increases with temperature, causing the output signal of the oscillator to vary with temperature as well as with the distance of the probe coil from an electrically conductive target material. The unwanted variation of the output signal with temperature can be somewhat compensated for by adjusting the operating frequency of the coil. Another procedure for compensating for temperature related alterations in coil resistance is to design the coil so that inductance of the coil increases when coil resistance increases. However, both this technique and the technique of adjusting coil frequency may not accurately compensate for changes in coil resistance concomitant with variation in the operating temperature of the coil.
Another drawback associated with attempting to compensate for PMT coil wires having temperature sensitive resistance characteristics is that the frequency utilized to operate the probe coil and oscillator must be high, often in excess of six hundred kilohertz. Also, since the operating frequency of the oscillator is dictated by the conditions necessary to compensate for temperature related resistance changes of the coil, it is generally not possible to select an operating frequency for a PMT which would provide optimal performance of the induction coil with respect to a particular electrically conductive target material.
Accordingly, it would be highly desirable to provide an improved inductive oscillator circuit in which the output signal varies with the inductance of a coil in the circuit and in which the inductive coil consists of a material having a resistance which remains relatively constant when the operating temperature of the coil varies.
It would also be highly desirable to provide an improved inductive oscillator circuit which would, when utilized in a proximity measuring transducer (PMT), permit the operating frequency of the PMT to be readily adjusted to obtain optimal operating conditions for the probe coil with respect to a particular electrically conductive target material.
Therefore, it is a principal object of the invention to provide an improved inductive oscillator circuit.
Another object of the invention is to provide an improved inductive oscillator circuit having a sine wave output which varies with the inductance of an inductive component in the circuit, the inductive component being comprised of a material having an electrical resistance which remains relatively constant when the operating temperature of the inductive component varies.
A further object of the invention is to provide an improved oscillator circuit which, when utilized in a proximity measuring transducer, facilitates adjustment of the operating frequency of the PMT to obtain optimal operating parameters for the PMT in relation to an electrically conductive target material.